The present invention relates to a light metering device used in cameras such as electronic still cameras, electronic video cameras and conventional film cameras.
Generally, lighting conditions under which a subject has a brightness considerably lower than its background are referred to as a rear-lighting or backlighting condition that is one of a number of undesirable conditions for proper exposure. Detecting backlight conditions is carried out by comparing the brightness of a subject located at the center area of an image field allocated for light metering with the brightness of the background included in the image field. This is well known in the art as a spot metering system.
Meanwhile, in recent years, it has become widely popular to equip photographic cameras with improved light metering devices in order to effect proper exposures even under backlight conditions. One example of such light metering devices is disclosed in, for example, Japanese Patent Publication No. 59-33,889. The light metering device includes a light metering system having a number of photo-electric elements arranged in a plane on which an image of a subject is focused. The light metering system compares the integrated brightness value of the subject detected in a spot metering manner with that of the whole image field to determine an existing lighting condition. If the difference of integrated brightness value of a subject relative to the background or the whole image field is larger than a predetermined value, the integrated brightness value of the spot metered subject is automatically selected to determine exposure. Otherwise, if the difference of integrated brightness value of the subject relative to the background or the whole image field is smaller than the predetermined value, the integrated brightness value of the whole image field is automatically selected to determine exposure.
Another example of such light metering devices can be found in, for example, Japanese Unexamined Patent Publication No. 62-96,824. in the light metering device, outputs representing brightness for several light metering areas within the whole image field are compared with a predetermined certain value. In this manner, a proper exposure can be determined even under backlight conditions by excluding outputs beyond the predetermined certain value.
In any light metering device mentioned above, it is quite difficult to place a principal subject at the light metering center area of the image field because of possibly moving the subject. It is also difficult to manually select a spot metering area in the image field on a principal subject. These difficulties result in an inaccurate light metering.